Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069; 5,593,838; and 5,603,351. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Micro pumps include electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The micro pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp. 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and then pumped into a bottom array of reaction cells. The array could be decoupled from the assembly and removed for incubation or analysis. When used as a printing device, the chemical reagent solutions are replaced by inks including cyan, magenta, and yellow dyes or of dispersions of cyan, magenta, and yellow pigments, and the array of reaction cells could be considered a viewable display of picture elements, or pixels, comprising dyes, pigments, or mixtures of dyes or pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of the receiver fibers pulls the dye from the cells and holds it in the receiver, thus producing a hard copy print, or reproduction, of the original scene.
Such a device includes a contact array print head. One way to render accurate tone scale would be to stock and pump a number of inks of different colors ranging from very light to dark. Another way to solve the tone scale problem is to print a very small dot of a particular color ink and leave white paper surrounding the dot. The human visual system will integrate the white and the small dot of dark color leading to an impression of light yellow, provided the dot is small enough. This is the principle upon which the art of color halftone lithographic printing rests. It is sometimes referred to as area modulation of tone scale. However, in order to provide a full tone scale of colors, a high resolution printer is required, with many more dots per inch.
Another way to render accurate tone scale for ink jet printers is described in U.S. Pat. No. 5,606,351 by Gilbert A. Hawkins, the disclosure of which is hereby incorporated by reference. U.S. Pat. No. 5,606,351 overcomes the above described problem by premixing the colored ink with a colorless ink in the correct proportions to produce a drop of ink of the correct intensity to render tone scale.
An alternate method has been proposed to render accurate tone scale by precisely timing the duration of contact of the ink with the receiver at each image pixel. In accordance, ink in each ink channel is brought into contact with the receiver by a micro pump located in the ink channel. The practice requires that termination of the printing of ink image pixels be precisely timed, since the ratio of printing time of one ink image pixel, during which ink from a corresponding ink channel is in contact with the receiver, to the printing time of other ink image pixels determines the fidelity of reproduction of the grayscale levels. Control of grayscale levels is well known in the art of image science to play an important role in image quality. Precise timing in turn requires that contact between the ink and the receiver be terminated precisely, which is difficult because of the tendency of the ink to remain in contact with the receiver due to the surface energy of the ink and receiver interface and to the cohesive energy of the ink, as is well known in the art of fluid dynamics.
One way of terminating contact of the ink with the receiver involves stopping or reversing the action of the pumps controlling the ink in each ink channel. This method is difficult and expensive and may result in imprecise timing because breaking contact between the body of ink in the microchannel and the ink contacting the receiver requires a very strong pump. Also, the exact location of the break point is uncertain if the ink channel is uniform. In addition, since the surface of the receiver is generally hydrophobic in order that it imbibe ink, prolonged time may be required to separate the ink in the body of the ink in the microchannel from that contacting the receiver. Also, it is difficult to turn off or on a plurality of micro pumps simultaneously due to the constraints of data handling and power consumption.
Another way of terminating contact of the ink with the receiver involves physically lifting the receiver from the substrate top surface, preferably rapidly, so that contact between the ink and receiver is broken simultaneously for all ink image pixels. However, separating the receiver from the top surface of the substrate containing the ink channels must be done in a manner which avoids smearing of the ink. Also, lifting the receiver to break ink contact rather than relying on reverse pumping of each microchannel may result in smearing and does not reliably leave the same liquid drop volume on each printed pixel.